It has been argued that the growth rate of exclusively breast-fed infants may be limited by their protein intake. This issue was examined using data from an intervention study in Honduras in which infants were randomly assigned to be exclusively breast-fed for the first 6 mo (EBF; n = 50), or to receive preprepared solid foods (including egg yolk) in addition to breast milk beginning at 4 mo (n = 91). Neither weight gain nor length gain from 4 to 6 mo differed between groups despite a 20% higher protein intake (as well as significantly higher intakes of iron, zinc, calcium, vitamin A, and riboflavine) in the latter group. The 20 infants with the highest protein intakes in that group were matched to 20 EBF infants on the basis of energy intake; protein intake was 1.46 +/- 0.09 versus 1.10 +/- 0.17 g/kg/d, respectively (p < 0.001), but growth rate did not differ between groups. Similarly, the 20 infants with the lowest protein intakes in the EBF group were matched (by energy intake) to 20 infants given solid foods; protein intake was very low in the former compared with the latter (0.81 +/- 0.13 versus 1.04 +/- 0.20 g/kg/d; p < 0.001), yet there was still no difference in growth. Infant morbidity was relatively low and did not influence the results. These analyses indicate that protein intake is not likely to be a limiting factor with regard to growth of breast-fed infants from 4 to 6 mo of age.